A. Field of the Invention
The present invention generally relates to electrodes for implantable cardiac stimulators, particularly defibrillators employing a "hot can" stimulus generator housing. More particularly, the invention relates to such stimulation electrodes having a coating that protects the electrode surface from oxidation, and still more particularly to such oxidation resistant coatings that contain a conduction-enhancing medium.
B. Description of the Related Art
Abnormal rhythms or arrhythmias can arise in the heart as a consequence of an impairment of the heart's electro-physiologic properties. Tachycardia, for example, is an arrhythmia characterized by rapid beating of the affected cardiac chamber which, in some instances, may lead to fibrillation. In other instances, fibrillation may arise in a diseased heart without the advance episode of tachycardia.
During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated, random contractions, quickly resulting in a loss of the blood-pumping capability of that chamber. During ventricular fibrillation (i.e., fibrillation occurring in a ventricle), cardiac output ceases instantaneously. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death of the patient will occur within minutes.
Because ventricular fibrillation is frequently caused by ventricular tachycardia, various methods and devices have been proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioversion. In the latter technique, the heart is shocked with one or more current or voltage pulses of considerably higher energy content than is delivered in pacing pulses from a pacemaker. Unfortunately, cardioversion itself presents a considerable risk of precipitating fibrillation, as a result commonly called "refibrillation."
Defibrillation, that is, the method employed to terminate fibrillation, generally involves applying one or more high energy "countershocks" to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections, thereby restoring the synchronized contractions of the atria and ventricles. Successful defibrillation requires the delivery to the heart of the afflicted person an electrical pulse containing energy at least adequate to terminate the fibrillation and to preclude refibrillation. Although high intensity defibrillation shocks are often successful in arresting fibrillation, they tend to precipitate cardiac arrhythmias, which themselves may accelerate into fibrillation, i.e., refibrillation. Additionally, high intensity shocks can cause permanent injury to the lining of the heart (myocardium).
In the conventional approach of external defibrillation, conducting paddles or electrodes are positioned on the patient's chest and electrical energy typically in the range of 100 to 400 joules is delivered to the chest area in the region of the heart. When fibrillation occurs during open heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium, and the energy required for defibrillation is considerably less, on the order of 20 to 40 joules.
More recently, implantable defibrillators have been developed for use in detecting and automatically treating ventricular fibrillation. In the last twenty years, a vast number of improvements in implantable defibrillators, including fibrillation detectors and high energy pulse generators with related electrode configurations, have been reported in the scientific literature and disclosed in patent publications.
Typically, electrodes for implantable defibrillators are made similarly to those developed for cardiac pacemakers, except defibrillation electrodes are larger than those used for cardiac pacing because a greater area of the heart tissue needs to be stimulated. These electrodes may be in the form of patches applied directly to the heart. The most common approach in the past has been to suture two patches to the epicardial tissue via thoracotomy. It has been theorized that electrodes with large surface areas are important for a wider distribution of current flow and a more uniform voltage gradient over the ventricles. Others have postulated that uniformity of current density is important since: (i) low gradient areas contribute to the continuation or reinitiation of ventricular fibrillation, and (ii) high current areas may induce temporary damage, that then may cause sensing difficulties, set-up areas of reinitiation of fibrillation, or even potentially cause permanent damage (new arrhythmias, decreased contractility, and myocardial necrosis).
For most patients, the best conventional devices and implantation methods are those that avoid surgical entry into the chest cavity and attachment of epicardial electrodes. Employing a less invasive surgical technique, one or more defibrillation electrodes are implanted proximate the pleural cavity and rib cage, and are used in combination with one or more coil electrodes positioned in the right atrium or right ventricle of the heart. This kind of defibrillator is described in U.S. Pat. No. 5,203,348, issued to Dahl, et al.
Stimulation of tissues requires that the charge be injected reversibly by a purely capacitive mechanism. In such a mechanism, the electrode behaves as a charge flow transducer whereby electrical discharge takes place in a uni-directional manner between media exhibiting different charge flow properties. The capacitive mechanism allows electrons to flow away from the stimulation electrode causing electrical charges at the electrode/electrolyte interface to orient themselves in order to cause a displacement current through the electrolyte. Since the electrolyte is an ionic medium, the slight displacement of the ions in reorientation creates a charge flow.
When irreversible chemical reactions begin to occur as a result of poor electrode selection or exceedingly high currents or other thermodynamic or kinetic limitations, the mechanism is no longer capacitive. Irreversible faradaic reactions may lead to water electrolysis, oxidation of soluble species, and metal dissolution from the electrode. In addition, some of the products of the reactions may be toxic. Neither gas evolution nor oxide formation reactions contribute to electrical stimulation of excitable tissue and stimulation energy is wasted in electrolyzing the aqueous phase of blood instead of carrying desirable charged species from one electrode to the other via the tissues. Because of the need for high energies in defibrillating the heart, higher currents are usually generated from the defibrillator than from a pacemaker. Under such circumstances, the efficiency of the electrode during high current generation is vital in not only reducing the defibrillation threshold, but in reducing the unwanted gas reactions or oxide formation. Excessive levels of gassing reactions may also lead to embolism in other vital organs such as the brain. Thus, stimulation electrodes should preferably allow a large charge flow across the electrode-tissue interface without the risk of irreversible faradaic reactions. Selection and design of the metal of the electrode is critical.
A metal of choice in electrode manufacturing has traditionally been titanium. On a fresh titanium surface, however, oxygen ions react with the titanium anode to form an oxide layer. Once a finite oxide thickness has been formed on the surface, polarization increases further. A point is reached when the oxygen ions reaching the surface of the titanium cannot be reduced further to form the oxide, and instead are reduced to elemental oxygen to form oxygen gas. The oxide film developed on the surface of a titanium electrode, either naturally or electrochemically, is irreversible. It cannot be reduced to the original metal by passing a charge in the reverse direction. Hence, it is clear that virgin titanium metal is a poor choice for electrode construction, since it forms a semi-conductive oxide on its surface before and even during the electrical stimulation. Platinum, and much more so stainless steel, have been shown to undergo irreversible dissolution during stimulation as well.
Very recently, the "hot can" type of implantable defibrillator has come into wide use. In this type of defibrillator, one of the defibrillation electrodes is formed by the stimulator housing itself, or a face, window or portion thereof. The unit is placed subcutaneously or extra-pericardially, such that the shock current will flow through the ventricular septum to a transvenous lead placed inside the heart. An example of this kind of defibrillator is described in U.S. Pat. No. 5,480,416 issued to Garcia et al.
Conventional titanium hot can electrodes, by their failure to be essentially reversible in redox reaction along their surfaces, permit build up of the irreversible electrochemical products upon the surfaces. This results in entrapment of ions, molecules, etc. derived from the serum or body tissue in closest contact with the electrode surface, such as chronic coagulation and fibrotic growth. This in turn results in a greater likelihood of coagulation of blood, fibrin formation and other clotting cascade moieties immediately next to the surface or entrapped therein. These entrapped and surface blocking particles further reduce the ability of the surface to pass a charge and lead to increased impedance across the electrode surface.
Titanium oxidation reactions are several times more likely in an oxidative environment than those of platinum or platinum alloys, but a thousand times less so than those of stainless steel. Unfortunately, due to the expense of platinum metal and the requirement for large amounts of metal in patch-type electrodes, production costs are too high for platinum electrodes. Because of the low current requirements for pacemaker electrodes, the need for high surface area may not be too critical. However, it becomes crucial to implement a high surface area electrode with low polarization so as not to cause too much gassing reactions. At this time, platinum is generally used only in pacemaker electrodes and transvenous defibrillation electrodes, and not in hot cans. The use of platinum in transvenous defibrillation electrode is either in the form of a planar, low surface area alloy as platinum/iridium, or as a smooth, low surface area platinum coating on a titanium substrate. In such latter cases, the use of maximizing the surface area by making the platinum porous is usually avoided because of the possibility of increased clotting on a porous surface. However, this exposes the possibility of increased current density and increased gassing reaction which may lead to possible embolism. Thus, even though oxidation problems are more prevalent in them, titanium is typically used for conventional pacing and defibrillator electrodes as the substrate material.
U.S. Pat. No. 5,601,607, issued to Adams, also discusses the problem with achieving optimal electrode function with conventional hot can, also called "active can," electrode designs due to oxidation of the can material, particularly with cans made of titanium or stainless steel. The '607 patent addresses the problem by coating the can with a noble metal, such as platinum, to reduce the effects of oxidation on the can material over multiple shocks.
A problem that is still not widely recognized at this time by those working in the area of cardiac stimulators, however, is that the housing ("can") material becomes porous as increasing numbers of shocks are administered, particularly with conventional titanium cans. As a result, the interfacial contact between the can surface and the body tissue diminishes and the path for adequate delivery of ionic current is correspondingly reduced, as illustrated in FIGS. 1A-B, and discussed in the Detailed Description, below. This results in high polarization at the can/tissue interface and leads to unacceptably high levels of defibrillation thresholds (DFTs).
In order to avoid passivation on the surface of titanium pacer electrodes, an iridium oxide ("IrOx") coating has been employed on the electrode of a pacing lead, as disclosed in U.S. Pat. No. 4,919,135 (Phillips, Jr. et al.), issued to Intermedics, Inc. The oxide formed by iridium is very stable, does not grow further, and is electrically conductive. In addition, it provides protection for the underlying metal and is reversible to aqueous based redox species, undergoing reversible redox reactions with species such as hydrogen ions and hydroxyl ions, leading to the formation of higher oxidation state surface oxides. U.S. Pat Nos. 4,717,581 and 4,677,989 teach deposition of iridium oxide onto the metal surface of an electrode. IrOx is rough, however, and it has been observed that rough-surfaced electrodes usually tend to cause scar tissue formation. In a surprising finding using the electrodes of the invention, it was found that the electrodes are capable of reducing the amount of both acute and chronic coagulation of blood surrounding the electrode. It is postulated that this reduction in the amount of coagulation of blood is a direct result of the reversible reduction-oxidation occurring over the enhanced electrically-accessible area of the electrodes. Where coagulation occurs immediately upon placement of the electrode in the tissue, it is said to be acute. Certain prior art electrodes have failed to be essentially reversible in redox reaction along their surfaces where the build up of the irreversible electrochemical products upon the surface results in entrapment of ions, molecules, etc. derived from the serum or tissue in closest contact with the electrode surface (chronic coagulation, fibrotic growth). This in turn results in a greater likelihood of coagulation of blood, fibrin formation and other clotting cascade moieties immediately next to the surface or entrapped therein. IrOx coated electrodes have not been employed for hot can electrodes.
An electrically stable can material that does not form an oxide in situ when subjected to numerous shocks, and provides a large charge flow, is needed for construction of the hot can defibrillator electrode. Such a material should also be very conductive and provide a very good tissue/can interface. An ideal electrode with a large charge flow can be obtained by selecting materials that: (a) are electrically stable; (b) undergo reduced unwanted reactions such as oxide formation or gassing reactions; (c) reversible; (d) reduce inflammation of the adjacent tissue and hence, provide thinner fibrous capsule at the electrode/tissue interface; (e) provide a continuum electrical interface with the tissue and electrode; and (f) are designed for high surface area. It would be beneficial to have a hot can defibrillator electrode that maintains a uniform defibrillation threshold over the useful lifetime of the unit.